The present invention can be best understood and appreciated by undertaking a brief review of the crude oil distillation process, and most particularly, the cost redemptive role delayed coking plays within that process.
In its unrefined state, crude oil is of little use. In essence, crude oil (a.k.a. hydrocarbon) is a complex chemical compound consisting of numerous elements and impurities. Such impurities include, but are not limited to sulfur, oxygen, nitrogen and various metals that must be removed during the refining process.
Refining is the separation and reformation of a complex chemical compound into desired hydrocarbon products. Such product separation is possible as each of the hundreds of hydrocarbons comprising crude oil possess an individual boiling point. During refining, or distillation, crude oil feedstock temperature is raised to a point where boiling begins (a.k.a. "initial boiling point, or "IBP") and continues as the temperature is increased. As the boiling temperature increases, the butane and lighter fraction of crude oil are first distilled. Such distillation begins at IBP and terminates slightly below 100.degree. F. The fractions boiling through this range are represented and referred to as the "butanes and lighter cut."
The next fraction, or cut, begins slightly under 100.degree. F. and terminates at approximately 220.degree. F. This fraction is represented and referred to as straight run gasoline. Then, beginning at 220.degree. F. and continuing to about 320.degree. F. the Naphtha cut occurs, and is followed by the kerosene and gas/oil cuts, occurring between 320.degree. F. and 400.degree. F., and 450.degree. F. to 800.degree. F., respectfully. A term-of-art "residue cut" includes everything boiling above 800.degree. F.
The residue cut possesses comparatively large volumes of heavy materials and two fundamental processes are employed to convert appreciable amounts of such residuals to lighter materials--thermal cracking and delayed coking. While thermal-cracking may be properly considered "the use of heat to split heavy hydrocarbon into its lighter constituent components," delayed coking should be considered "severe thermal cracking" and occurs within a coke drum after a coker feedstock has been heated in an apparatus referred to as a coking heater, or "delayed coker charge heater." An improved delayed coker charge heater and process serve as the focus of the instant invention.
Delayed coking processes and heaters are well known in the art and have been discussed and disclosed, for example, in U.S. Pat. No. 5,078,857, invented by M. Shannon Melton and issued Jan. 7, 1992 (hereafter referred to as "Melton"). Melton and prior art references cited therein are hereby provided to disclose and distinguish said art from the novel improvements embodied and afforded by the instant invention.
Delayed coker charge heaters as represented by the present art employ, at best, a double fired, single row of feedstock encapsulating tubes which are heated in a "radiant" heat section of a delayed coker charge heater to a pre-defined temperature. Such single row, double fire technology fails to create any measurable convective heat transfer to the coke feedstock while the feedstock resides within the heater's radiant heating section. While radiant heat may be properly viewed as "heat transferred by the emission of waves from a fixed point or surface", convective heat is to be considered as "heat transferred via the circulatory motion that occurs in a fluid at a nonuniform temperature owing to the variation of its density and the action of gravity." Failure to introduce and contain significant quantities of flue gas for convective heat transfer within the "radiant heat section(s)" of today's double fired delayed coker charge heaters represents a present art design deficiency. A deficiency manifested in the present art, and remedied by the present invention.
Succinctly stated, as the temperature of flue gas decreases from flame temperature to exit temperature within the radiant heat section of a delayed coker charge heater, the present art fails to provide boundaries, or a channel, within which flue gas may be re-circulated to create convective heat transfer and improve radiant efficiency. Prior art reference, FIG. 1, is included herewith to illustrate a typical double fired coker heater configuration as represented by the present art. Similar configurations may be examined when reviewing Melton and relevant art cited therein.
In the present invention, a double row heating conduit creates a channel between the two interior boundaries of the conduit. This channel provides for significant backside convective heat transfer to the conduit as the cooled flue gas flows downwardly, within the channel toward the bottom of the radiant heat section of the delayed coker charge heater. Absent such backside convective heat transfer the double row, double fired heating conduit (synonymously referred to as "coil") would require 1.5/1.2 or 25% more surface area to create an equivalent average radiant flux rate to that realized when employing a single row, double fired coil. With backside convective heat transfer, however, the afore-stated ratio is reduced to 1.275/1.2, or only a 6.25% increase in radiant surface area. In reality, when the circumferential conductive heat transfer in the tube wall is taken into account, the difference is reduced to essentially 0 and the same number of tubes and surface area can be used.
In addition to advancing convective heat transfer, the double row, double fired heating conduit provides for substantial savings in coker construction materials. As an example, for thirty four-inch nominal tubes in a single row configuration, on eight-inch centers, the coil height would be nineteen feet four inches. For thirty four-inch nominal tubes in a double row configuration on twelve-inch triangular centers, the coil height is fourteen feet six inches. When utilizing a double row configuration, the net savings in firebox height equals four feet ten inches. Thus, the double row configuration results in significant savings in steel, refractory and other materials of construction.
The afore-stated convective heat transfer to the coil actually improves the radiant efficiency. As flue gas circulation rates are greatly increase in the radiant sections a more uniform vertical heat flux profile is created, thus reducing the temperature of the flue gas exiting the coker heater's radiant section and increasing radiant section thermal efficiency.
Hence, given the deficiencies of the present art and improvements afforded by the instant invention, what is needed is an improved process and article of manufacture to advance the performance of delayed coker charge heaters by introducing convective heat transfer to the interior portion of a double row, double fired, heating conduit transporting coker feedstock within the radiant heat section of said heaters.